This invention relates generally to furnaces which utilize fuel oil or waste oil as fuel and relates, more particularly, to the means by which fuel is preheated prior to its introduction into the combustion zone of such a furnace for burning.
Prior art furnaces which burn fuel oil or waste oil in a combustion zone commonly include a nozzle block assembly utilizing an atomizing nozzle through which air and oil are conducted into the combustion zone for burning and a fuel pump and air compressor for delivering fuel and air, respectively, to the nozzle. It is desirable in some instances that the oil is preheated prior to its introduction into the combustion zone, and for purposes of preheating the oil, known schemes exist. For example, in U.S. Pat. No. 5,372,484 having the same assignee as the instant invention, a nozzle block assembly is described wherein an electrical resistance heater is supported within the nozzle block for heating the nozzle block (primarily by radiant heat emitted from the element) and air routed through the block. The heated nozzle block, in turn, heats the oil routed therethrough. Thermostats mounted within the block monitor the block temperature and prevent operation of the oil pump unless the temperature of the nozzle block is within a preselected temperature range.
Another preheating scheme, such as is described in U.S. Pat. No. 4,487,571 having inventors in common with the instant invention, involves the mounting of a positive temperature coefficient (PTC) thermistor on one side of the nozzle block. The oil contained within the fuel passageway provided within the nozzle block is heated by the transfer of heat from the PTC thermistor through the body of the nozzle block.
It is known that if the oil is preheated to an unacceptably high level, the oil may carbonize (or burn partially) prior to its introduction into the combustion zone of such a furnace and thereby reduce the effectiveness of the oil for heating purposes. Along the same lines, if the oil is heated through a surface (e.g. a metal surface) which is much hotter than the temperature to which the oil is desired to be heated, the oil may carbonize and an undesirable build up of carbon will collect upon the surface, and the carbonized oil and carbon build up can adversely effect furnace operation. On the other hand, if the oil is not heated to a high enough temperature prior to its introduction into the combustion zone, the oil is likely not to burn completely within the combustion zone thereby wasting energy and producing pollution from unburned hydrocarbons.
It would be desirable to provide an improved oil preheating scheme for an oil-burning furnace of the aforedescribed class which is less likely to overheat, and thereby carbonize, oil prior to its introduction into the combustion zone of the furnace.
Accordingly, it is an object of the present invention to provide a new and improved system for preheating fuel prior to its introduction into the combustion zone of a furnace of this class while controlling the power consumption of the preheating system for preheating the fuel.
Another object of the present invention is to provide such a system which is effective in operation, is safer in some respects, and is less likely to overheat oil than are preheating schemes of the prior art. draws power on an as-needed basis to maintain the oil at the desired temperature.
In another aspect of the invention, the improvement comprises shutoff means associated with the fuel passageway disposed upstream of the nozzle block assembly and responsive to the internal pressure of the air passageway for shutting off the flow of oil along the fuel line upon shutdown of furnace operation.